1. Technical Field of the Invention
This invention relates to an ignition apparatus for an internal combustion engine arranged in such a way that no ignition spark is generated when the crank shaft of the engine is rotated in the reverse direction during starting of the engine.
2. Description of the Prior Art
When starting an internal combustion engine, an ignition switch is normally operated to drive a starter which rotates the crank shaft. The ignition coil is energized for a part of the period of one revolution of the crank shaft and is then de-energized. Upon de-energization of the ignition coil, an ignition spark is generated at the ignition plug to explode the mixed gas thereby maintaining the rotation of the crank shaft.
In a conventional ignition apparatus, a sensor is provided for sensing two specific angular positions in one revolution of the crank shaft for achieving the energization and de-energization of the ignition coil. When the sensor senses one of the angular positions the ignition coil is energized, and when the other angular position is sensed the coil is de-energized to generate a spark at the ignition plug.
FIG. 1 is a circuit diagram of an example of such a conventional ignition apparatus. In this figure, a sensor 1 for sensing angular positions of a rotary member, such as, for example, a crank shaft CS, of an internal combustion engine EN generates, in synchronism with the rotation of the crank shaft CS, an output which falls from a high level to a low-level at a first angular position .theta..sub.1 for every rotation of the crank shaft CS and another output which rises from the low-level to the high level at a second angular position .theta..sub.2. Thus the output of the sensor 1 is low from .theta..sub.1 to .theta..sub.2 (first state) and high from .theta..sub.2 to .theta..sub.1 (second state). The output terminal of the sensor 1 is connected to the base electrode of a first transistor 2, the emitter electrode of which is, in turn, connected to the ground, the collector electrode of which being connected to the positive end of a battery 3 through a resistor 4. The collector output of the first transistor 2 is connected to the base electrode of a second transistor 5. The emitter electrode of the second transistor 5 is grounded and the collector electrode thereof is connected to the positive end of the battery 3 through the primary winding of an ignition coil 6. Connected between the output end of the secondary winding of the ignition coil 6 and the ground is an ignition plug 7.
FIG. 2 shows waveforms of an output voltage a of the sensor 1, a collector voltage b of the first transistor 2, a collector voltage c of the second transistor 5 and a secondary output voltage d of the ignition coil 6. The operation of the ignition apparatus of FIG. 1 will be described below with reference to FIG. 2.
When the sensor 1 senses the first angular position .theta..sub.1 of the crank shaft CS at the time t.sub.1 the output of the sensor 1 goes from the high level to the low level and thus the first transistor 2 turns OFF, making the second transistor 5 ON (conductive) and thereby allowing a current to flow through the primary winding of the ignition coil 6. The sensor 1 senses the second angular position .theta..sub.2 of the crank shaft CS at the time t.sub.2. At this time, the output of the sensor 1 goes from the low level to the high level to turn the first transistor 2 ON. Accordingly, the second transistor 5 is turned OFF (or cut off) to stop the flow of current through the primary winding of the ignition coil 6 so that a secondary high output is generated in the ignition coil 6 to generate a spark at the ignition plug 7. At subsequent times t.sub.3 and t.sub.4 similar operations are performed. From the time t.sub.3 at which the sensor 1 senses the first angular position .theta..sub.1, a current flows through the primary winding of the ignition coil 6, and at the time t.sub.4 at which the sensor 1 senses the second angular position .theta..sub.2, a spark is generated. From the time t.sub.5, a current flows through the primary winding of the coil 6, as described above. Since the second angular position .theta..sub.2 is approximately at the compression top dead center of the engine EN, the compression pressure within the cylinder of the engine EN is increased as the crank shaft CS rotates in the forward direction from .theta..sub.1 to .theta..sub.2. If the driving force of the engine EN is insufficient, however, it may happen that the momentum of the crank shaft CS is overcome by the compression pressure, the crank shaft CS fails to pass over the top dead center, and rotates back toward the bottom dead center. Let's suppose that, before the sensor 1 senses the second angular position .theta..sub.2, the crank shaft CS begins to be rotated in the reverse direction at the time t.sub.6. The above-described rotation will hereafter be referred to as the forward rotation and the rotation in the direction opposite thereto is referred to as the reverse rotation. Then, the crank shaft CS passes through the first angular position .theta..sub.1 in the reverse direction at the time t.sub.7. This results in the fact that the output of the sensor 1 at the time t.sub.7 is reversed from the low level to the high level. This is an operation reverse to that which takes place at the time t.sub.5 at which the crank shaft CS passes through the first angular position .theta..sub.1 and the output of the sensor 1 goes from the high level to the low level. Since the output of the sensor 1 goes to the high level at the time t.sub.7, the first transistor 2 is turned ON and thus the second transistor 5 is turned OFF. At the time T.sub.7, therefore, the current flowing through the primary winding of the ignition coil 6 is cut off to allow a spark to be generated at the plug 7. This spark would encourage the possibility of reverse rotation of the crank shaft CS, leading to a danger of engine damage.